Erm, you mean you don't know how to edit a post? Well, it's quite simple, really:

Step 1: Look at your post. On the top right corner you'll find two buttons, the right one says "quote" and the left one says "edit".
Step 2: Click the "edit" button.
Step 3: Make the additions you wanted to make.
Step 4: Tick the "add edit line" box if necessary.
Step 5: Click the "submit modified post" button.

It's just that double posts within minutes of each other are a bit annoying, you see. Especially if the double posts are nearly identical apart from five words stuck on at the end. Besides, editing takes exactly the same amount of time as making a new post, sometimes even less, so you really should do it.